LDL receptor relative with 11 ligand-binding repeats (LR11) is an LDL receptor-like protein having a structure characteristic to the LDL receptor family and a molecular weight of about 250 kD (Patent Document 1, Non-Patent Document 1). In addition to membrane-bound type LR11, soluble LR11, a segment cleaved by protease is known as LR11 (Non-Patent Document 4). It is reported that little or no expression of LR11 was observed in normal vascular wall cells, but expression thereof was observed specifically in thickened intima smooth muscle cells (Non-Patent Document 2). Moreover, it is reported that expression of LR11 was promoted in response to proliferation of cultured smooth muscle cells, leading to secretion of soluble LR11 to the culture liquid, and that thickening of vascular intima, which would be caused by migration and proliferation of smooth muscle cells, is inhibited in a cuff injury mouse model by functionally impairing LR11 gene in the mouse by developmental engineering (Non-Patent Document 3). Furthermore, the present inventors previously found that the blood soluble LR11 levels of arteriosclerosis patients are significantly higher than those of healthy subjects, and reported that the blood soluble LR11 could be used as a new marker for arteriosclerosis (Non-Patent Document 5, Patent Document 2).
Hitherto, there is a known method for determining soluble LR11 through separating soluble LR11 from a sample by use of an insoluble carrier to which a chaperone molecule, a receptor associated protein (RAP) having affinity to LR11, has been bound, followed by performing SDS-PAGE and Western blotting to detect LR11 through immunostaining by use of an anti-LR11 antibody (Non-Patent Documents 5, 6). However, since the known method includes cumbersome steps such as separation of soluble LR11 from a sample, the method is not practical for application to clinical tests or the like.
The present inventors previously tried to establish an immunological assay method employing an anti-soluble LR11 antibody, as a simple and practical soluble LR11 determination method. However, when a biological sample such as a serum sample was analyzed, an unidentified interfering substance present in the biological sample (hereinafter may be referred to simply as an “interfering substance”) was found to interfere with correct determination of soluble LR11.
Therefore, the present inventors extensively studied on means for eliminating the effects of interfering substances on the immunological assay method. As a result, the inventors found that, when a biological sample such as a serum sample is mixed with a specific surfactant such as N-acyl-N-methylglucamine, and the thus-treated sample is subjected to an immunological assay, soluble LR11 present in the sample can be correctly determined in a simple manner while eliminating the effects of interfering substances, and the inventors previously filed a patent application (Patent Document 3).